The image quality in the case of X-ray tubes, in particular in the case of microfocus X-ray tubes, is impaired by the fact that an interfering bright circular disc often appears in the generated X-ray image. This circular disc is caused by scattered X-radiation which forms when electrons strike the diaphragm body of a lens diaphragm of the X-ray tube. The lens diaphragm is referred to as diaphragm in the context of this application. As the diaphragm body must be high-temperature-resistant and therefore consists in particular of metal, when the electrons strike the diaphragm body short-wave X-radiation forms which penetrates the target and projects an image of the diaphragm pinhole onto the image receptor when higher energies of the electrons are used.
DE 10 2006 062 454 A1 describes a microfocus X-ray tube which solves this problem by means of a coating of the diaphragm. The metal of the diaphragm is coated with a material with a low atomic number in order to reduce the stray radiation. A disadvantage here is that coatings are usually only possible in the micrometre range. For example, a carbon coating of approximately 4 μm is possible. The penetration depth of the electrons is, however, much more than 4 μm in the case of high energies, as a result of which the electrons penetrate all the way into the metal and generate stray radiation. Moreover, the diaphragm is exposed to high thermal loads. In the case of coated diaphragms this often leads to a peeling of the coating.
A collimator for electron beams in an X-ray tube is known from U.S. Pat. No. 3,227,880. This collimator is constructed in two parts. Its part near the electron source—first part—consists of a metal with a low atomic number, for example aluminium, and its part far from the electron source second part consists of a metal with a high atomic number, for example lead. In principle, the collimator aperture passing through the two parts is formed such that its area is greater at the entrance side near the electron source than at its exit side far from the electron source; it thus narrows in the beam direction of the electron beam. The collimator aperture has a first aperture part (which is formed in the first part) and a second aperture part (which is formed in the second part). Both aperture parts in each case separately have a truncated cone-shaped surface area. These aperture parts can be formed either widening or narrowing in the beam direction. The first aperture at the end of the first aperture part far from the electron source is formed smaller than the second aperture at the end of the second aperture part near the electron source, with the result that there is a step in the beam direction, which extends into the electron beam. Alternatively, in the case of aperture parts in each case narrowing in the beam direction, the first and second apertures can also be the same size. In both embodiments, electrons can strike the second part and generate stray radiation there.
A diaphragm for an applicator to be used in electron irradiation therapy is known from DE 10 2011 005 450 A1. This diaphragm is constructed in a three-layer arrangement, wherein the layer facing the irradiation direction of the electrons consists of a first metal with a first atomic number, which is smaller than a second atomic number of a second material of the middle layer, which is in turn smaller than a third atomic number of a third material, a layer facing away from the irradiation direction of the electrons. The diaphragm aperture is formed in the shape of a cylinder jacket.